1. Field of the Invention
The present invention relates to electric power steering systems (EPSs) and gain setting methods thereof.
2. Description of the Related Art
A column assist electric power steering system (hereinafter referred to as the “column EPS”) in which an electric motor and a speed reduction mechanism are placed in a column portion is known as an electric power steering system. The column EPS includes a steering wheel, a steering shaft, an intermediate shaft, a steering operation mechanism, an electric motor, a speed reduction mechanism, etc. The steering shaft is formed by an input shaft coupled to the steering wheel, an output shaft coupled to the intermediate shaft, and a torsion bar coupling the input shaft and the output shaft. The electric motor is coupled to the output shaft via the speed reduction mechanism.
As used herein, the term “column” refers to a portion that is formed by a steering wheel, an input shaft, a torsion bar, and an output shaft. The term “power column” refers to a portion that is formed by a steering wheel, an input shaft, a torsion bar, an output shaft, a speed reduction mechanism, an electric motor, and a control device for the electric motor. The term “common column EPS” refers to a column EPS that does not have a function to compensate for friction of a speed reduction mechanism.
In common column EPSs, motor torque (assist torque) generated by an electric motor is transmitted to an output shaft via a speed reduction mechanism. The assist torque thus transmitted to the output shaft is transmitted via an intermediate shaft to a steering operation mechanism including, e.g., a rack and pinion mechanism. For example, the speed reduction mechanism is a worm gear mechanism that is formed by a worm gear and a worm wheel. Since large friction is generated in the speed reduction mechanism, this friction may affect the response of the steering system to steering input.
Methods for compensating for friction that is generated in a speed change mechanism have been developed in order to improve the response of the steering system to the steering input. The easiest friction compensation method is a method in which a certain frictional force is added to an assist torque command value according to the sign of the steering speed.
Japanese Patent Application Publication No. 2003-170856 (JP 2003-170856 A) discloses a method for compensating for friction based on an assist torque command value calculated based on steering torque detected by a torque sensor, and the detected steering torque. Specifically, the magnitude of the frictional force that is generated in the speed reduction mechanism is estimated based on the assist torque command value. The sign of the frictional force in the speed reduction mechanism is determined based on the detected steering torque. The frictional force in the speed reduction mechanism is estimated in this manner. The estimated frictional force in the speed reduction mechanism is added to the assist torque command value.